DE2819538A1 - IMAGE PLAYBACK DEVICE - Google Patents

Description

Patent attorneys Dipl -Ing. Shark 1978 German-lng Dipl.-Chem. G. Quieter E. Prince Dr. G. Hauser Ernsbergerstrasse 19 8 Munich 60 3.

THOMSON -CSF

173} vol. Haussmann

75008 PARIS / France

Unaer Ze lohen:! E 3097 Image playback prevention

The invention relates to image display devices, with which a single image or a sequence of images can be made visible, in the form of an electrical signal which is commonly called a video signal.

Such a picture display device can be in a television system, particularly for projecting television images onto a large screen.

It is known to use a cathode ray tube for displaying images, but it becomes disadvantageous considered that such a tube would be excessively large in size to obtain a substantially planar image must have.

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Numerous devices have therefore already been vorgesohlagen that can be used to replace the cathode ray tubes are determined, but the results so far have always been disappointing. Sometimes the structure is complicated and the brightness is low, as with the Piasma screens, as described by Yoshifumi Amano in the journal "IEEE transactions on electron devices", Vol. ED-22, No. 1, January 1975. Sometimes the response speed is slow, so that it can be used with a television system is not possible, as is the case with the numerous Plussigkri stall devices that are still being experimented with today.

In SR-PS 2 275 087 there is an image display device described in which a thermo-electric effect in a liquid crystal layer is applied, the writing of the image by scanning the layer takes place with the help of an infrared beam, which is carried out by a Laser is generated and deflected by electro-optic or acousto * optic deflectors.

This light source and deflectors are both bulky and expensive. The Naohteil, which is on strongest noticeable maoht, however, consists in that the applied deflection must be slow enough to allow a point-wise local change in the phase of the Liquid crystal layer is possible under the action of the heat supplied by the infrared beam. Such thermal information is too slow to allow the device to be used in a television system.

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The object of the invention is to increase the speed of writing the image into a picture a material that is a solo thermo-electric Has effect; this is done through the use of devices with which the picture can be inscribed line by line, so that the whole sour one Line for the simultaneous reaming of all points of the line.

Further features and gate parts of the invention result from the following description of exemplary embodiments the invention based on the Zeiohnung. In the newspaper demonstrate:

fig. 1 is a partially broken away view of a display cell according to the invention,

Fig. 2 is a schematic top view of an image display device according to the invention;

3 shows the circuit diagram of the device in FIG included sampling circuit,

4- the circuit diagram of the device of FIG contained control circuit,

FIG. 5 shows the general position of a projection device, showing the use of the device from Fig. 2 allows

6 is a partial sectional view of a modified embodiment of the image display device according to FIG of the invention and

Fig. 7 is a schematic representation of another Projection device.

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In Fig. 1 a very simple display point is shown, which, for the sake of clarity, is limited to two rows and two columns in which an image is displayed can, which consists of four sparks arranged in a square.

A thin liquid crystal layer 101 is sandwiched between a silicon oxide (SiO ₂ ) 102 obtained by oxidizing a silicon substrate 103 and a second substrate 104 made of sapphire. It is held in its position by spacers not shown.

Row conductors 105 and 106, for example made of aluminum, are applied to the silicon oxide layer in such a way that they are in contact with the liquid crystal. They serve as heating resistors when they _{are fed via connections L 1} and L _2, which are brought to an electrical potential with respect to a common ground connection M, whereby the temperature of the liquid crystal can be increased locally over the entire length of the connected row conductor.

The liquid crystal heated in this way is preferably such that it has a smectic A phase and has a nematic phase. Good results have been obtained with the compounds of the biphenyloyanide family, used alone or in admixture were, and in particular with Ootyl-Biphenyloyanid, the is commonly called COB, and this is the following Structural formula has:

809845 / 10OS _PKafM M.

COB has a transition from the smectic phase to the nematic phase at 32.5 ° O and a transition from the nematic phase to the isotropic phase at 40.5 ° C.

When the display cell is thermostatically a few degrees below the smectisoh-nematic transition is maintained, can duroh the temperature increase as a result of the heating caused by the row conductor brought the liquid crystal along the row conductor into the isotropic phase in a very short time will.

When the voltage applied to the row conductor used is interrupted, the heat is essentially dissipated via the substrate 103, which is a good conductor of heat. The Si0 ₂ -Sohioht 102, which in turn is a poor heat conductor, on the other hand, limits the speed of this heat dissipation. Therefore, if it is desired that the response time of the system is short, the surface 102 must be thin so that rapid cooling takes place. However, one is subject to restrictions in this regard, since the heat dissipation also occurs during the heating process, which necessitates an increase in the heating power, which causes the risk of the heating conductors melting.

Good results have been obtained in a cell which has a more complicated structure than that of fig. 1, and contained 600 rows and 800 columns, aluminum conductors having a diameter of 1 μm, a width of 15 μm and a length of 2 cm, spaced 5 μm apart and applied to a SiO ₂ -sohioht of 3 μm Sioke. These row conductors had a resistance of about 50 Ω and enabled one when applied

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Supply voltage of 25 V to achieve the transition a 15 mm thick COB bead from the nematic into the isotropic phase in less than 60 µs. She also returned to the smectic state in less than 60 / us.

When sioh the liquid crystal in such a short time cools down, it takes on a so-called focal cone structure, which gives it a diffuse appearance, in contrast to the completely translucent state of the liquid phase and the normal homeotropic phase. If then, during this cooling process, an electrical field - which corresponds to a voltage - closes the liquid crystal layer is applied, depending on the value of the field strength, nan receives a more or less scattering structure. In this cell the liquid crystal is perfectly scattering for a voltage of 10 Y, while for a tension of 20 Y it is completely transparent, with the percentage of permeability being substantially linear with the tension changes.

A solo effect, the one-ear rubbing into a material under the effect of the electric field during cooling is called thermo-optisoh. He is not specifically for liquid crystals, in particular in the manner of COB, and is also formed in particular with lithium niobate, which is doped with iron, in which the writing manifests itself in a change in the refractive index. Man could also start in the same way from a scattering liquid crystal and heat it in bring a more or less strongly scattering state.

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Liohtdurohläeeige conductors 107 and 108 - for example made of tin oxide SnO ₂ - are sprayed on the surface of the substrate 104 which is in contact with the liquid crystal layer 101, perpendicular to the row conductors 105 and 106. In the example described, these column conductors have the same width and the same spacing as the row conductors, so that essential display points are obtained which lie essentially in a square on the intersection points of the row conductors and column conductors. The SnO ₂ -Sohioht has a low Dioke, so that a good Liohtdurohlässigkeit is obtained, but the value of the Dioke is not critical, since the delivered Stroa is practical Hull.

When a DC or Veohsel photovoltaic is applied to a column conductor or to both column conductors, an electric field is created between these or each column conductor and the row conductors, one end of which is grounded. This field has the same value everywhere only if a row conductor is not fed with a bit of a heating current, since otherwise the potential of the row conductor τοη decreases from the side connected to the connection L ₁ or L ₂ to the side connected to ground. This is irrelevant, however, since the phenomenon of ear rubbing only takes place when it cools down, i.e. when the line conductor is not fed with a heating current.

So you can write four points in the liquid crystal layer 101 at the crossing points of the row conductors 105 »106 sit the column conductors 107, 108, their Scatter supply vos value Hull up to a maximum value can be controlled continuously.

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It should be noted that the reaming is necessarily carried out line by line, because when a line conductor heats the crystal, it erases the previous inscription both at the point of intersection with one column conductor and at the point of intersection with the other column conductor, so that it is necessary to re-write the corresponding points at these two crossing points. if conversely, the two row conductors would heat at the same time, the same column conductor would simultaneously receive the same information at the points of intersection with the enroll in both line leaders.

The cell shown in Fig. 1 is essentially of an experimental type. It has the implementation of the diagram in Pig. 2, which has 600 row conductors LA ₁ to IAg ₀₀ on a first substrate 203 made of silicon and 800 column _{conductors CA 1} to CAg ₀₀ on a second substrate 204 made of sapphire. These substrates, together with the spacer 201, define a liquid crystal layer. The values of 600 lines and 800 columns have been chosen so that, taking into account the time lost in the line rewind and the field rewind, a television program can be reproduced with the European standards of 625 lines and 25 images per second. These standards therefore correspond to a line duration of 64 Ms, which is compatible with the numerical values given above, according to which the heating and cooling times per line are less than 60 us.

A television video signal is connected through a terminal YS a sampling circuit 205 is applied. This circuit enables the video signal to be sampled and stored

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the attast values thus obtained under the control by means of a clock signal which arrives via the connection HC and a line synchronizing signal which is applied via the connection HL. In this way, 800 samples of the video signal are available every 64 Ma for a duration of 64 yus at the 800 connections C ₁ to C ₈₀₀ , which have been taken in the course of the duration of a scan line in such a way that the beginning of the line at connection C ₁ and the end of the line is at the end of the line Cqqq. These terminals are connected to the column conductors CA carried by the substrate 204.

The circuit 205 is formed directly on the substrate 204 by means of integration according to the method known under the designation SOS. In the course of this integration, both the connections C ₁ to C ₈₀₀ and the column conductors CA ₁ to CA ₈₀₀ are formed by vapor deposition. This solves the problem of making the interconnections, which is known to be a major problem in all matrix access devices. Namely, once the manufacturing masks have been formed, the entirety of the connections can be made in a single simple and inexpensive operation included in the manufacturing process of an integrated assembly. The dimensions of the circuit are not an obstacle to the value of the devices (e.g. a cathode ray tube) that can be replaced by the device of the invention.

For rubbing in the scanned and stored lines a row conductor, for example the row conductor LAj, is heated during the time in which the corresponding

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Video signal arrives at port 205; the heating of the row conductor is then switched off, and the scanned voltages are applied _{to the column conductor CA during the time in which the video signal corresponding to the row IAj +} -] is arriving at the connection 205. During the same time line IA-J _{+1 is} heated to prepare it for reaming; the same process is repeated in the manner indicated for a non-interlaced image.

In order to obtain this sequence of successive heating of the rows, the row conductors are fed with the aid of the control circuit 206, which in turn is formed on the substrate 203, preferably by integration. In a process step of this integration, the connections L ₁ to LgQO ^{and α} * ^θ row conductors IA ₁ to LAgQQ are formed, which simply represent an extension of the connections, since they can be formed from the same material, for example aluminum.

Additional experience has shown that one can get a heating rate of the lines that one Heating time of less than 10 yus is equivalent by adding the increased power applied to the row conductors, and a cooling rate that has a cooling time of less than 50 its corresponds by taking the thickness of the SiOg-Sohioht decreased. One can then carry out the full reaming cycle for the duration of one line.

Control circuit 206 receives clock signals at terminal HL in the rhythm of the sequence of lines in the video signal, at terminal Hf clock signals in the rhythm of the sequence of fields

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and a power supply voltage for the heating of the row conductors LA. The remaining power supply connections for circuits 205 and 206 as well as the common ground at the right end of the row conductors are not shown.

3 shows an exemplary embodiment of the sampling circuit 205.

The clock signals HC arrive at a counter 301. In order to achieve a horizontal resolution that is essentially the same as the vertical resolution, a clock rate has been selected for these signals that is equal to the 880-fold of the clock rate of the line synchronization signals, where nan assumes that the fraction of the scanning line 10 that was lost during synchronization # amounts to. The counter 301 thus runs to the rhythm of the signals HC, and in order to ensure its synchronization, it is systematically reset to zero by the line synchronization signal arriving at the connection HL. If nan assumes as an example that the lost part of the video signal is half before and half near the line synchronization pulse, it can be seen that the 800 samples to be taken from the video signal lie between the counts 40 and 839 of the counter. Each of these counter readings is decoded and results in a digital signal which appears in sequence at the 800 connections SC * ₀ to SCgxQ.

These connections are each connected to one of 800 Monentanwertspeiohern EA ₁ to EA ₈₀₀ , which continuously receive the video signal VS. So while the counter 301

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continues from count 40 to count 839, key the instantaneous value memory EA under the control through the counter the video signal of the series naoh, and they store the samples taken in this way

The outputs of the instantaneous value stores EA are each connected to the inputs of 800 further instantaneous value stores EB ₁ to EBg ₀ Q, which simultaneously receive the line synchronization signal from the terminal HL in parallel at their scanning control input. In this way, when the counter 301 starts up again, the sampled values contained in the instantaneous value memories EA are transferred to the instantaneous value memories EB, which they store for the duration of the following line, while the instantaneous value memories EA are then available again. The outputs of the instantaneous value stores EB directly form the connections C ₁ to C800 » ^at which the sampled values of the video signal taken during the previous line are available for the duration of a line.

Fig. 4 shows an embodiment of the control circuit for the Pail that the written image interlaced from two express images of 312.5 lines each consists.

The line synchronization signals are from the connection HL on a counter 401 applied; this counter also receives the field sync signals from the terminal HX, whereby it can be reset to zero at the beginning of the field. The counter 401 thus runs in the rhythm of the signals HL around. As an example, assume that 12.5 lines

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by giving the partial image synchronization lost, of five whole cells at the beginning of the field and seven whole lines at the end of the field, while the half line required for the interlace is at the end of the odd field and at the beginning of the even field, it can be seen that the 300 lines of a field to be written in correspond to the counter readings 5 to 304- of the counter. Each of these counter readings is decoded and results in a digital signal, that of the row next to the 300 connections SLn to SL ™, appears.

Each of these connections leads to a pair of AND circuits in a group of AND positions ET ₁ to ETg ₀ Q. The output of each of these positions is connected to the base of an npn translator. The collectors of these transistors are connected in parallel to the connection YT (heating power supply for heating the row conductors), and the emitters are each connected to one of the connections L ₁ to Lg ₀₀ .

To select the even and odd fields, the field synchronization signal from the terminal HT to a Flip-flop 402 applied, which changes its state in time with these signals; one output of this flip-flop is parallel to the inputs of all of the And-So positions connected with an even index and the other output in parallel with the inputs of all AND circuits with odd index. Thus, the counter opens the 300 and-so positions one after the other with an odd index, which in turn unlock the 300 corresponding transistors, thus sequentially the 300 odd-numbered ones Row conductors are heated, and then the same opens Count the 300 and so positions with an even index one after the other, to get the 300 even number one after the other

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Heat row conductor. So that the line jump is correct occurs, the flip-flop 402 is forcibly brought into the state at the beginning of the odd-numbered fields in which the odd ANDs be opened. For this purpose the line synchronizing pulses and the field sync pulses from the terminals HL and HT to an AND circuit 403 created which only opens when these pulses are synchronous, which only occurs at the beginning of an odd number Partial image of the pail is. The output of the AND circuit is connected to a forcing input of the flip-flop 402 tied together.

Such a picture display device is not self-luminous and can only be used with external lighting. In the example shown, this external lighting is effective by reflection on the metal layer forming the row conductors, this reflection depending on the state of the liquid crystal is more or less diffuse. if the row conductors would not have sufficient reflectivity, for example due to the use of another Materials, one would use a dielectric mirror applied to the row conductors.

If you use a material other than a liquid crystal, for example the aforementioned lithium niobate, are the changes brought about by rubbing in the optical properties are not always directly visible, so that measures must then be taken to make them appear. If the refractive index changes due to birefringence (in the case of lithium niobate) one can, for example, use the device

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view through a circular polarizer. the Dimensions are small because of the application of thermal phenomenon, but they allow direct Viewing from a short distance. An interesting application is to put the device in a helmet to be installed, as it can be used in particular by an aircraft pilot.

To get a picture of larger dimensions, can to use a projection device. This arrangement must be adapted to the diffusion device used so that sufficient contrast is achieved. So you can use a Schlieren optics, as in the device known under the name Eidophor. It However, it is also easily possible to use an objective with a small numerical aperture which works in reverse to the Schlieren optics, namely eliminating the scattered rays as much as possible and only allowing the directly reflected rays to pass through.

Such a projection device is sohematisoh shown in FIG. A light source 501 illuminates with a converging beam via a condenser an image display cell 503 of the type described above at a relatively small angle θ with respect to on the normal Z on the cell. The light thrown back from the cell is, depending on the inscribed Image, divided into two parts, one part of which is a directly reflected beam which is symmetrical at the same angle θ with respect to the incident one The bundle is reflected, while the other part is scattered light that is scattered back into the reohte half-space

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2B19538

will. A projection objective 504, the opening of which is just sufficient to receive the reflected beam, is generated an enlarged image of the cell 503 on a projection screen 505. The respective inclinations and positions the cell 503, the objective 504 and the projection screen 505 are chosen so that the image is not distorted will; this is done according to a method that is usually used in cinematographic projection where the projector is seldom in the axis of the projection screen. ! The lens that a small one Has an opening, has a great depth of field and shows little Aberrations, which facilitates its positioning with respect to the cell and the projection screen.

It should be noted that, in contrast to, for example, cathode ray tubes, the image is written continuously except for the line written during the time it is heated. One obtains thereby a good contrast and a memory effect which is applicable in the case that one, for example wants to reproduce an image as a still image with the incoming video signal switched off.

A liquid crystal cell tends to have a smectic phase to become slightly dispersive in the long run if one does not take measures that encourage a spontaneous return of the liquid crystal into an ordered, homogeneous and therefore completely transparent phase. To remedy this phenomenon, for example, the surfaces of the substrates can be coated with a thin Cover layer of surfactant that matches the liquid crystal

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is and a slow and spontaneous reorientation of the Liquid crystals allowed. For every liquid crystal at least one suitable surfactant is known; the product known as silane has been used very extensively and is particularly suitable for the case that the aforementioned liquid crystal is used COB is used. In this case, nan loses under the effect of spontaneous reorientation, the memory effect, because the inscribed image is erased a duration of a few tens of milliseconds. So you lose some of the contrast when using in a television system, because the picture is partially deleted from one field to the next, but that by the Suppression of the aforementioned disturbing effect obtained gain is greater than the loss, so that the balance is positive.

In such a case, the use of Schlieren optics is more advantageous than the spontaneous deletion of the In the case of optics of the type shown in FIG. 5, the image results in a brightening which is more unpleasant than that Darkening caused by the optics of the soles will.

In Fig. 7 is an embodiment of a projection device shown, in which such a sole optics is used. A light source 701 is illuminated Via a condenser 702 and a semitransparent mirror 706 an image display cell 703 with a converging incident bundle, which is coaxial to the normal Z on the cell. The reflected light is divided into two parts, namely a directly reflected bundle F, which is reflected in itself in a specular manner

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and in turn lies coaxial with the axis Z, and in a scattered light component D. A projection objective 704 from a large opening receives the scattered light component D - at least partially - and generates it on a projection screen 705 is an enlarged image of cell 703.

A screen 707, which is preferably located in front of the objective 704 close to the focus point of the bundle F, absorbs this bundle, so that the image produced on the projection screen is exclusively due to the scattered light component D is generated.

Pig. Fig. 6 shows in section part of another embodiment of an image display device according to the invention. A liquid crystal layer 601 is inserted between a transparent substrate 604, for example made of glass, and a silicon substrate 603. The substrate 603 carries an insulating layer 602 made of silicon oxide SiO 2, which has been obtained by oxidation and on which the heating row conductors 605 are applied, which are visible in cross section. The row conductors are embedded in a silicon oxide layer 609, which has been obtained, for example, by sputtering. The column conductors 608 are applied to the layer 609, only one of which is visible in longitudinal section in the drawing. This makes it possible to integrate the control circuit for the column conductors into the substrate 603 and thus to use ordinary glass for the substrate 604. The scanning voltages of the video signal are applied between the column conductors and a transparent ground counter electrode 610, which _{consists for example of SnO 2} and is applied to the inner surface of the substrate. In this embodiment is on

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the column conductor 608 applied a dielectric mirror, which consists of strips 611 that are parallel run to the row conductors 605. These stripes each reflect one of the three primary colors for an additive color mixture, for example the colors red, green, blue; each of these strips is a row conductor assigned, and they are arranged in a regular permutation. This device enables thus the reproduction of a color image, but the measure used for this purpose can also be applied to the device of Fig. 1 can be applied. This measure can also be implemented in other forms; for example the strips 611 can run parallel to the column conductors 608, or they can simply go through Filtering act and be applied to the substrate 604. You can also use other color mixtures, for example two-color mixtures that are adapted to special systems.

Another embodiment is, in the device of FIG. 2, a substrate 204 of ordinary To use glass and to implement the control circuit 205 with thin-film transistors of the TFT type.

In order for the devices to work properly, it is appropriate to have the liquid crystal layer - and therefore practical the whole cell - thermostatically regulating to a temperature that is slightly (1 or 2 ° C) below the temperature the transition from the smectic phase to the nematic Phase lies.

A good measure to do this is to hit the surface of the permeable substrate a transparent one

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Apply electrode and one through this electrode to let electrical current flow, its value in Depending on the temperature of the substrate is regulated, the temperature, for example, with an in the substrate embedded thermistor is measured.

The devices described above can be applied similarly to those used in the manufacture of integrated circuits Technology can be realized. When used, they only require a supply of voltages and signals of the usual type and illumination by a light source, which can optionally be ambient lighting, or the introduction iu a simple projection device.

The devices according to the invention are suitable for Use for every reproduction of moving or still images that are scanned according to a standard, which is adapted to the picture to be displayed and in no way limited to the official television standards which have been given in the preceding description.

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Claims (1)

Patent attorneys Dipl.-Ing. Dipl.-Chem. Dipl, -Ing. E. Prince - Dr. G. Hauser - G. Leiser Ernsbergerstrasse 19 8 Munich 60 May 3, 1978 THOMSON - CSP 173, vol. Haussmann 75008 PARIS / France Our sign; M 3097 Claims Device for reproducing images scanned in the form of at least one grid consisting of lines and represented by a video signal, with a layer of a material which can be rubbed on by a thermo-electrical effect, characterized by means for heating the layer along the lines to be recorded at least up to its dissolving temperature, where each line as a whole is briefly heated after the previous line and before the following line, and duroh facilities with which to enter the layer while it is cooling down at the same time along all lines electric field can be applied which is a sample of the portion of the video signal which corresponds to the line to be recorded, the value of the field being so large that it enables thermo-electrical writing. 309845/1008 lei / Gl ORIGINAL INSPECTED 2. Device according to claim 1, characterized in that that the material of the layer is a liquid crystal. 3. Device according to claim 2, characterized in that that the liquid crystal before heating a smectic Has phase and at the solution temperature in the isomorphic phase passes over, and that on cooling applied electric field when returning to the smectic phase depending on its value one more or less pronounced non-scattering state. 4. Apparatus according to claim 3, characterized in that that the liquid crystal between the smectic phase and the isomorphic phase has a nematic phase. 5. Apparatus according to claim 4, characterized in that that the liquid crystal belongs to the biphenyloyanide family heard. 6. Device according to one of claims 1 to 5, characterized characterized in that the material of the Sohicht between two substrates is included, which are the heating devices and carry the means for applying the electric field. 7. Apparatus according to claim 6, characterized in that that the heating devices have electrical resistances in the form of heating conductors applied to one of the substrates exhibit. 809845/1008 8. Apparatus according to claim 7, characterized in that the means for applying the electrical Field electrodes in the form of column conductors that run perpendicular to the heating conductors and are applied to the other substrate, the entirety of the heating conductors serving as a counter electrode. 9. Apparatus according to claim 7, characterized in that the means for applying the electrical Field electrodes in the form of column conductors that run perpendicular to the heating conductors and are applied to the same substrate as these, as well as a translucent one applied to the other substrate Counter electrode, and that an insulating layer insulates the heating conductors from the electrodes. 10. Device according to one of claims 7 to 9, characterized in that the heating devices have a power supply control hold have, under the control duroh external synchronization signals an electrical heating voltage in sequence to each heating conductor applies, and that the power supply organ in the form of an integrated circuit in which the heating conductor supporting substrate is formed. 11. The device according to claim 10, characterized in that the substrate carrying the heating conductors is made of silicon consists. 12. Apparatus according to claim 8 or 9, characterized in that the means for applying the electrical Field contain a one-eared posture, which is under the 809845/1008 Controlled by external synchronizing signals, the video signal scans, stores the signals corresponding to the scan of a line, and applies an electrical one-ear rubbing voltage between each of the electrodes and the common counter-electrode, which corresponds to the same Column corresponding stored sample value, and that the mono-ear recording is designed in the form of an integrated circuit in which the electrodes supporting substrate is arranged. 13. Device naoh claims 8 and 12, characterized in that the substrate carrying the electrodes made of sapphire. 14. Device naoh claims 9 and 12, characterized in that the substrate carrying the electrodes consists of a translucent amorphous material. 15. Device naoh one of claims 7 to 14, characterized in that devices are provided, which enable the color of an external light reflected from the device to be selected, and that these facilities delimit successive groups of three heating conductors each, one each correspond to another color and enable the reproduction of a color image through additive color mixing. 16. Device naoh one of claims 8 to 14, characterized in that devices are provided, which enable the color of an outer light reflected from the device to be selected, and 809845/1000 that these bodies are successive groups of limit three electrodes each, which each correspond to a different color and enable the reproduction of a color image duroh additive color mixing. 17 '· Device according to one of claims 1 to 16, characterized duroh devices that allow a spontaneous return of the liquid crystal into an ordered homogeneous phase. 18. Device according to one of claims 1 to 16, characterized in that the surfaces in contact with the liquid crystal layer have no treatment which would cause the liquid crystal to spontaneously return into an ordered homogeneous phase, so that the inscribed image can be used until another Image remains saved. 809845/1008